This invention relates generally to fire alarm systems, and more particularly, to methods and apparatus for calculating power conditions at devices prior to installation.
In general, alarm systems include several notification appliances that are joined to common power lines extending from a control panel. Notification appliances may be wired as notification circuits by routing the wiring from the control panel to each device in succession. Alternatively, when wired as signaling lines, the wires may spoke off to form multiple wiring runs or branches. Forming branches in signaling line circuits is also referred to as “T-Tapping”.
Notification appliances have one or more alarm indicators (e.g., strobes and horns) to notify people in the area of an alarm condition. Each notification appliance has a power operating range within which the device operates. A predefined or predetermined power (e.g., voltage and current) operating range may be set by a manufacturer or a regulatory body such as Underwriters Laboratories (UL). Different types of notification devices may have different voltage and/or current operating ranges. If the voltage is below the operating range, the appliance may not operate properly, producing visible signal intensities or sound pressure levels below the levels of the designed system.
Under normal operating conditions, an AC power provides a primary source of power to the control panel. A secondary source of power, such as storage batteries, is typically available. Fire alarm codes, such as NFPA 72, require that the system be operable for a minimum period of time when using the secondary power source, such as 24 hours, 60 hours or other length of time specified by the Authority Having Jurisdiction (AHJ). As the batteries are discharged, the output voltage supplied to the notification appliances decreases. Therefore, the system is required to operate with the power source at less than the rated input voltage, such as 85% of the rated input voltage.
Many of the notification appliances in use are “constant power” loads. When input voltage is reduced, the current increases, and thus the current draw of a notification appliance at reduced voltage is higher than when at the normal operating voltage. The increase in current draw at lower voltages also results in greater line loss than when operating under normal conditions.
Prior to installation, voltage drop calculations are obtained for the alarm system. Calculating actual voltage drop for appliances is extremely difficult, especially for T-Tapped circuits. Therefore, T-Tapped circuits are limited to known distances and the circuit is considered as a single line during calculations. A lump sum calculation is done for each wiring run to calculate the worst-case voltage drop based on low-battery operation. This method often results in severely limiting wiring distance, which is undesirable.
Therefore, a need exists for an automated method for calculating the voltage drop of appliances and other devices installed on single and multi-branch circuits. Certain embodiments of the present invention are intended to meet these needs and other objectives that will become apparent from the description and drawings set forth below.